A backlight illuminates a liquid crystal (LC) based display panel to provide light distribution over the entire plane of the LC display (LCD) panel. Typical direct-lit LCD backlights consist of individual fluorescent lamps placed in a reflecting cavity to directly shine light upwards towards and through the LCD panel.
A typical direct-lit LCD backlight has a diffuser plate to hide the individual lamps. The diffuser plate is typically filled with light-scattering particles, has a transmission of only about 55% and a transmission haze of over 99.6% to drastically scatter the light so that the individual lamps cannot be seen. On top of the diffuser plate is a “bottom diffuser” that is typically a plastic film coated with spheres and a binder, which aids in hiding the bulbs, but also turns or collimates the light somewhat in the direction of the viewer. Often a prism film is arranged on the diffuser plate, where the prism film has prisms running in a horizontal direction (direction parallel to the orientation of the lamps) to collimate the light strongly in the vertical direction (direction in the plane of the prism film and perpendicular to the horizontal direction). Typical applications for direct-lit backlights are in televisions, where it is acceptable to collimate the light vertically since viewers typically do not view from above or below the screen, while it is typical to not collimate horizontally since it is common to view the screen from side angles.
Diffuser plates are used in direct-lit flat panel displays because of their bulb hiding ability. It is possible to increase the absolute hiding power of a diffuser plate by adding more light scattering particles. However, there is a tradeoff between absolute hiding power and luminance. As more scattering particles are added, the scattered light travels a greater distance through the diffuser plate with a corresponding increase in absorption. Secondly, as more scattering particles are added, the diffuser plate reflects more light with the additional probability of being absorbed. Therefore, increasing the scattering particles of a diffuser plate will increase the absolute hiding power but only after a significant loss in luminance due to absorption. The current objective in the display industry is to manufacture thinner displays and to remove bulbs; therefore it is desirable to consider alternative backlight designs that will achieve both the luminance and absolute hiding power in Ultra-Thin displays and Ultra-Thin displays with fewer bulbs.